This invention relates to an antistatic coating and its method of preparation.
The metal structure of modern aircraft, the speed and external surface area of which continue to increase, constitutes a conducting path allowing the flow of electrostatic charges generated by triboelectricity, the ionisation of gases thrown out by the engines and the generation of an electric field. However, non-conducting elements, such as hatches, miscellaneous fairings and radomes, consisting of layers of insulating synthetic fibres coated with resin and then polymerised, resist this flow by encouraging the accumulation of electrostatic charges on their surfaces. The same is true of metal surfaces coated with insulating paint, since the paint acts as a very thin non-conducting element.
Charges building up on the surfaces of these non-conducting elements raise the latter to a potential different from that of the aircraft's metal structure. This difference in potential increases more or less rapidly until it reaches the breakdown voltage. A series of discharges then takes place, the electrical energy of which interferes with the correct functioning of equipment carried on board by generating radio-electrical interference. This process is accelerated and amplified by bad weather.
In order to alleviate this interference, which may considerably attenuate the quality of navigational aids and reduce the efficiency of telecommunication means, one must endeavour to render the surface of aircraft equipotential by means of coatings capable of rendering these insulating surfaces conducting or antistatic.
However, there are non-metallic elements distributed on the surface of aircraft, particularly radomes protecting the antennae, which cannot be made too conductive under penalty of making them opaque to electromagnetic radiation by modifying their radio-electrical transparency.
In the case of these radomes, therefore, a paint must be used which, although a conductor, is free from metal particles the distributing power of which is considerable. Such a non-metallic paint must allow the flow of static charges and exhibit a surface resistivity which is proportioned judiciously so as not to modify the radio-electrical transparency of the radome.
Furthermore, once it has hardened this paint should retain its characteristics virtually permanently since, generally speaking, the natural aging of such coatings tends to make them more conducting with the result that these coatings tend to become more conductive than is desirable after prolonged use. They would then interfere with, instead of improving, the good functioning of the antenna protected by the radome.
Studies carried out by the applicants in this field have resulted in a precise definition of the desired value of the surface resistivity of coatings applicable to radomes and similar surface elements. This surface resistivity, measured by the standard US-MIL-C-7439 and expressed as "ohms per square", must be, for maximum efficiency, between 5 and 100 megohms per square and be stabilised at that value.
However, it is found that such a stabilisation of surface resistivity is extremely difficult to obtain since, as has been said, commercial antistatic coatings have a tendency to become more and more conductive in the course of time. It follows, on the one hand, that a coating which is initially too conducting would scarcely improve with time, while, on the other hand, a coating which is initially too insulating would tend, on aging, to attain the correct resistivity at a certain moment before becoming in its turn too conducting. Obviously, it is not possible to make the best of such products.